(1) Field of the Invention
The present invention relates to a signal amplifier circuit suitable to reduce distortion of a pulse width caused due to input offset when a pulse signal is reproduced in, for example, a receiving portion in an optical communication system.
(2) Description of the Related Art
In general, in a receiving portion in a telecommunication system or the like, a signal amplifier circuit is used to reproduce a pulse signal from a very weak analog signal so as to shape a waveform of a signal attenuated at a time of transmission.
The signal amplifier circuit is desired to have a wide dynamic range with respect to an input signal level.
As a signal amplifier circuit meeting such a requirement, there has been known an ATC-limiter amplifier circuit including an ATC (Automatic Threshold Control) circuit (hereinafter referred to as ATC circuit) to detect a peak value and a bottom value of an input signal so as to set an intermediate value between the values as a threshold value, and a limiter amplifier to perform limiter amplification of the input signal depending upon the threshold value.
FIG. 30 shows one illustrative configuration of a signal amplifier circuit including an ATC-limiter amplifier circuit.
As shown in FIG. 30, a signal amplifier circuit 100 includes an ATC-limiter amplifier circuit 101 including an ATC circuit 102 and a limiter amplifier circuit 106, and another limiter amplifier circuit 107 on the side of a subsequent stage of the ATC-limiter amplifier circuit 101.
The ATC circuit 102 detects a peak value and a bottom value of an input signal to set an intermediate value between the values as a threshold value, and includes a peak detecting circuit 103, a bottom detecting circuit 104, and a voltage dividing circuit 105.
Here, the peak detecting circuit 103 detects the peak value of the input signal to output the value as a "1" level, and the bottom detecting circuit 104 detects the bottom value of the input signal to output the value as a "0" level. Further, the peak detecting circuit 103 and the bottom detecting circuit 104 respectively include a diode, a holding capacitor, and an amplifier circuit which are unillustrated.
Further, as shown in FIG. 30, the voltage dividing circuit 105 includes resistances 105A, 105B connected in series between an output terminal of the peak detecting circuit 103 and an output terminal of the bottom detecting circuit 104, and finds a divided voltage level of the peak value from the peak detecting circuit 103 and the bottom value from the bottom detecting circuit 104 by resistive division of the resistances 105A, 105B, thereby setting the divided voltage level as a threshold value (threshold level). Typically, a voltage division ratio is set to one-half so that the threshold level becomes an intermediate value between the "1" level and the "0" level.
Further, the limiter amplifier circuit 106 takes as inputs the input signal and the threshold value from the ATC circuit 102, and performs the limiter amplification of the input signal with center at the threshold level, thereby performing limiter processing such that output amplitude can be held constant throughout a range of the input signal.
Further, the limiter amplifier circuit 107 takes as input a signal (differential signal) obtained through the limiter amplification by the limiter amplifier circuit 106 of the ATC-limiter amplifier circuit 101, thereafter performing differential amplification of the input signal.
According to such a configuration, in the signal amplifier circuit 100 shown in FIG. 30, when the signal is input, the peak value and the bottom value of the input signal are detected in the ATC circuit 102 of the ATC-limiter amplifier circuit 101, and the intermediate value between the values is set as the threshold value.
Subsequently, when the threshold value and the input signal are input into the limiter amplifier circuit 106, the limiter amplifier circuit 106 performs the limiter amplification of the input signal with center at the threshold level, thereby holding the amplitude of the input signal constant.
Further, when the limiter amplifier circuit 107 takes as input the signal obtained through the limiter amplification by the limiter amplifier circuit 106 of the ATC-limiter amplifier circuit 101, the limiter amplifier circuit 107 performs the differential amplification of the input signal.
As set forth above, according to the signal amplifier circuit 100 shown in FIG. 30, the ATC circuit 102 sets the threshold value depending upon the input signal. Therefore, when an input level of the signal is varied, the threshold value can be varied while following the variation. Hence, it is possible to obtain a good output waveform over the wide dynamic range.
However, in the signal amplifier circuit 100 shown in FIG. 30, the peak detecting circuit 103, the bottom detecting circuit 104, and the limiter amplifier circuits 106, 107 have offsets inherent therein. Therefore, the threshold level output from the ATC circuit 102 is deviated from an original threshold level by an offset value.
Here, the peak detecting circuit 103, the bottom detecting circuit 104, and the limiter amplifier circuits 106, 107 respectively have a maximum offset value of ten millivolts. Consequently, accumulation of the offsets of the respective circuits results in an appreciably large offset value with respect to the amplitude of the input signal.
For example, when a signal (see reference numeral S) as shown in FIG. 31(a) is input, as shown in FIG. 31(b), the ATC circuit 102 detects a peak value level P and a bottom value level B from the input waveform S to set the original threshold level to a one-half level depending upon the values (see reference numeral H).
However, in the ATC circuit 102, the offset value (see reference numeral OF) present in the limiter amplifier circuit 106, the peak detecting circuit 103 and the bottom detecting circuit 104 is added to the threshold level, resulting in outputting a threshold level T.
Accordingly, the limiter amplifier circuit 106 performs the limiter amplification by using the threshold level T which is deviated by the offset value. Thus, there is a problem of degradation in the pulse width of a signal output from the limiter amplifier circuit 106 as shown in FIG. 31(c) (see reference numerals Q, R).
FIGS. 32(a), (b) show eye patterns of signals output from the limiter amplifier circuit 106. When the limiter amplification is performed in the limiter amplifier circuit 106 by using a threshold to which no offset is added, an output signal indicates the normal eye pattern as shown in FIG. 32(a). In contrast with this, when the limiter amplification is performed by using the threshold value to which the offset is added, the output signal indicates the abnormal eye pattern as shown in FIG. 32(b).
Further, in case of the input signal having lesser amplitude, the threshold level may exceed an amplitude range of the input signal by the addition of the offset value. In such a case, the limiter amplification of the signal cannot normally be performed in the limiter amplifier circuit 106.
In view of the above problems, there is a possible method in that a feedback loop (offset compensation feedback loop) is mounted in the signal amplifier circuit 100 to start from an output terminal of the limiter amplifier circuit 106 of the ATC-limiter amplifier circuit 101, and a feedback control is made by using an output signal from the limiter amplifier circuit 106, thereby compensating for the offset present in the circuits.
Here, the feedback control in the offset compensation feedback loop possibly includes one method in which comparison is made between peak values of the output signals from the limiter amplifier circuit 106 to make the feedback control so as to provide equal peak values, and the other method in which comparison is made between mean values of the output signals to make the feedback control so as to provide equal mean values.
However, it is possible to employ, only within narrow limits, the latter method in which the feedback control is made by using the mean values of the output signals because of problems in that, for example, the method is affected by a mark ratio of the signal. Thus, the former method is naturally employed in which the feedback control is made by using the peak values of the output signals.
However, a predetermined level or more is limited in the output signal from the limiter amplifier circuit 106 to cause a loss of amplitude information, and the output signal has a constant peak value irrespective of the threshold level. As a result, there is a problem in that it is impossible to employ the method of making the feedback control by using the peak values of the output signals.
Hence, there has been desired a signal amplifier circuit in which the offset can be compensated while taking advantage of the ATC circuit 102.